Marconi M361
  • Power: Battery driven: 2V LT & 120V HT
  • Wavebands: tuneable from 100KHz to 4.5MHz across 6 switched bands
    • 100 - 235KHz
    • 215 - 510KHz
    • 0.460 - 1.08MHz
    • 1.0 - 2.25MHz
    • 2.0 - 4.5MHz
    • 3.5 - 8.5MHz
  • Valve lineup: 4 total
  • Released: c.1938

This is an early TRF set designed for marine use that was also adopted as 'Admiralty Pattern 361' for naval use.

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Thanks to John Beaumont G3NGD for information on this venerable old set.

The Marconi Type 391 and 394A TRF Receivers were initially used on fishing trawlers in from 1938 omwards. They were used in conjunction with Marconi Transmitters, Type 517 and 527.

To determine the power requirement it is important to check the valve types as versions with 2V, 4V and 6V filaments exist. This can be done using a Valve Data Book. The M361 Receiver was usually supplied with 2-volt valves as standard and these were directly heated.

The battery supply terminals are situated on the front of the receiver and suitably labelled.

Both H.T.3 and H.T.2 terminals should be joined together and to the positive terminal of a 120 volt H.T battery.
Terminal H.T.1 should be connected to a 72 volt H.T.battery.

The grid bias battery, which is connected externally, supplies the negative bias required by the output valve. The positive terminal of the grid bias battery must be connected to the L.T. negative. For an anode voltage of 120 volts on the output Valve, then the grid bias voltage required is approximately 3 volts.

It is very important to check the valve types as well as the heater voltage. If directly heated valves are used they must be supplied with a Direct Current supply. It is very difficult to provide this from a mains power supply. There have been many circuits in for pure d.c. rectification from the mains supply, but there is always a risk of 'a.c.ripple' on the output audio. Later receivers Marconi 394C/12 and 394C/30 used indirectly heated valves which don't give rise to the same problems.

The receiver consists of two stages of high frequency amplification, a detector on which reaction is available and one output stage. The output stage may be either of the pentode type or the power valve type, according to the duties required of the receiver.

The aerial coupling is arranged to suit the various wave-ranges of the receiver, and gives improved results over the basic 394 receiver. The three tuned stages are all carefully ganged, and a single dial only is required for fine tuning within any given range. There are four wavebands covered on most versions of the
receiver, but the frequency ranges can vary on different models.

It is important to mention however, that the fourth wave band range is obtained, not by the use of a fourth tuning coil, but by adding a parallel capacitor of 0.0004 microfarad capacity in parallel with the normal tuning capacitor of approximately 0.0005 microfarad maximum capacity. This parallel capacity must be added on all three tuned circuits, and consequently it was necessary to have the three 0.0004 microfarad capacitors accurate to within 1 per cent of one another to ensure correct ganging.

Reaction is obtained by feed-back through the 0.0003 microfarad variable capacitor to a coil magnetically coupled to the tuning coil in use in the anode of the second screened grid valve - this gives good amplification. Volume control is obtained by means of the grid leak bias potentiometer, which varies
the negative voltage on the grids of the variable-mu screened grid valves.

A local-distant switch on the left-hand side of the receiver reduces the input from strong signals if required.
A small lamp has been included in the front of the panel, which illuminates the dial when the receiver is switched on.

Last updated 26th July 2003